Medical diagnostic ultrasound devices today play a crucial role in patient examination and diagnosis. The most common modes of diagnostic ultrasound imaging include B and M modes (used to image internal structures), Doppler, and color flow (the latter two used primarily to image flow characteristics, such as blood flow in blood vessels). In conventional B mode imaging, an ultrasound scanner transmits ultrasound signals into the body over a range of angles and focused at a desired depth. The ultrasound scanner creates images in which the brightness of a pixel corresponding to an angle and depth is based on the intensity or strength of ultrasound signal reflections from internal structure.
The color flow mode reveals the velocity of blood flow toward or away from the transducer, as determined by the measured frequency shift between transmitted and received ultrasound signals. Blood flow toward the transducer results in a higher frequency ultrasound signal reflection, while blood flow away from the transducer results in a lower frequency ultrasound signal reflection (measured at the transducer). The magnitude of the frequency shift is related to the velocity of blood flow. The frequency shift measurement techniques are also the basis of Doppler mode. However, whereas Doppler mode displays velocity versus time for a single selected sample volume, color flow mode displays hundreds of adjacent sample volumes simultaneously, all superimposed on a B-mode image and color-coded to represent velocity in each sample volume.
In the past, however, measured flow velocity has been obtainable more easily from larger vessels that carry fluid flow primarily in one direction (such as those found in the arms and legs). In part, this limitation stems from the fact that frequency shift (i.e., Doppler shift) is an angle dependent phenomenon. In other words, frequency shift varies from substantially zero when the transmitted ultrasound signal is incident normal to blood flow, to a maximum when the transmitted ultrasound signal is incident parallel to blood flow. The limitation also stems from the fact that smaller vessels are often blocked (or acoustically shadowed) by larger objects in the body, particularly from certain angles. Thus, regions of the body with many small blood vessels often suffered from poor imaging.
Some areas of the body have high blood vessel density (and thus have blood vessels oriented at many angles) or include blood vessels branch that branch in many directions. These areas often present near normal angles of incidence to transmitted ultrasound signals. As noted above, near normal angles of incidence reduce frequency shift between transmitted and reflected ultrasound signals to near zero. Thus, the ultrasound signal reflections contain little information about fluid flow in such areas of the body, thereby preventing accurate colorflow and Doppler imaging of the area.
Thus, a need has long existed for an improved method and apparatus for ultrasound imaging which overcomes the difficulties noted above, and others previously experienced.
A method is provided for multi-angle compound flow imaging that includes the steps of receiving at an ultrasound transducer first and second (in general, N) ultrasound signal reflections from a target. The ultrasound signal reflections are oriented at a first and second angle with respect to an ultrasound transducer normal. The method then jointly evaluates information derived using the first and second ultrasound signal reflections to determine a display result for the target and displays the display result, for example, in an angiogram.
The joint evaluation may include determining a first velocity associated with blood flow in the target using the first ultrasound signal reflection, determining a second velocity associated with blood flow in the target using the second ultrasound signal reflection, and selecting the greater of the first and second velocity as the display result. Similarly, the joint evaluation may include determining a first velocity associated with blood flow in the target using the first ultrasound signal reflection, determining a second velocity associated with blood flow in the target using the second ultrasound signal reflection; and assigning the average of the first and second velocity as the display result. Alternatively, the joint evaluation may select one of the first ultrasound signal reflection and the second ultrasound signal reflection as a greatest energy ultrasound signal reflection and set as the display result blood flow velocity information derived using the greatest energy ultrasound signal reflection. The foregoing are examples only however, and other joint evaluations of the received ultrasound signals may also be used.
An apparatus for ultrasound imaging device is provided that includes an ultrasound transducer for receiving first and second ultrasound signal reflections from a target. The first and second ultrasound signal reflections are oriented at first and second angles with respect to an ultrasound transducer normal. A processor is coupled to the ultrasound transducer and jointly evaluates information derived using the first and second ultrasound signal reflections to determine a display result for the target. A display coupled to the processor for displays the display result, for example, in a colorflow mode, Doppler mode, or angiogram.
The processor may, for example, determine a first velocity associated with blood flow in the target using the first ultrasound signal reflection, determine a second velocity associated with blood flow in the target using the second ultrasound signal reflection, and select the greater of the first and second velocity as the display result. As another example, the processor, during the joint evaluation process, may determine a first velocity associated with blood flow in the target using the first ultrasound signal reflection, determine a second velocity associated with blood flow in the target using the second ultrasound signal reflection, and set the average of the first and second velocity as the display result. Alternatively, the processor may select one of the first and second signal reflections as a greatest energy ultrasound signal reflection and set as the display result blood flow velocity information derived using the greatest energy ultrasound signal reflection.
The method and apparatus may determine that the most accurate blood flow information, for example, consistently returns along a particular angle. That angle may be selected for a predetermined duration as the only angle along which blood flow velocity information is extracted from ultrasound signal reflections. Furthermore, the ultrasound signal reflections may result from individually transmitted ultrasound signals at numerous angles, or from a single centrally transmitted ultrasound signal.
The ultrasound imaging method and apparatus of the preferred embodiment provide an enhanced ability to accurately image fluid flow, particularly in those areas of the body that are dense with blood vessels (in comparison, for example, to an arm or leg). Thus, as examples, the kidneys, thyroid gland, brain, testes, breast, and even areas of Neo-Vascularization (including that cause by malignant tissue) may be visualized. The present techniques further allow selection between relatively higher frame rate, enhanced imaging in a lower frame rate examination mode (such as an angiography mode), or persistent reception of ultrasound signal reflections at selected angles that provide the most accurate fluid flow information.